Negative feedback amplifiers are widely used to provide low distortion gain in many electronic systems. The widespread use of such amplifiers has led to the availability of one of the most general purpose negative feedback amplifiers, the operational amplifier, in the form of low-cost monolithic integrated circuits. Some electronic systems, notably servo controllers and audio power amplifiers, require that the negative feedback amplifier deliver a substantial amount of power to the load. Accordingly, in addition to the large number of general purpose operational amplifiers commercially available (which are usually limited to less than half a watt of output power), several manufacturers have developed operational amplifiers (often monolithic) with output powers in excess of fifty watts. An example of such a (monolithic) operational amplifier is the LM3886 from National Semiconductor Corporation, which can deliver 60 watts of power, and is primarily intended for audio applications.
One feature, which is considered important for such high power amplifiers is the ability to withstand the effects of (usually unintentional) loads which attempt to draw more power from the amplifier than it can safely provide. An accidental short circuit from the output to ground is an example of such an excessive load. One protection system commonly used is to limit the maximum output current of the amplifier to a value independent of the load. This avoids fusing of devices internal to the amplifier, which gives short-term protection in the case of a short circuit or other overload. Unfortunately, even with current limiting, the presence of such an overload for an extended period of time can cause failure of the amplifier due to excessive internal power dissipation and subsequent overheating. To overcome this problem, a second form of protection is frequently incorporated, which senses the temperature of the amplifier and removes the ability of the amplifier to supply output current when a safe temperature has been exceeded. This is known as thermal shut down. It is thus common for commercial amplifiers to feature both current limiting and thermal shut down, and the LM3886, for example, includes both types of protection.
For some applications, the output power provided by monolithic amplifiers such as the LM3886 is insufficient, and it is therefore desirable to find a way of obtaining more power from such amplifiers. Since power is the product of voltage and current, increasing either can theoretically raise the power. This invention is concerned with a method of increasing the overall current available to the load in a system employing a commercially available amplifier (such as the LM3886). Such techniques are in common use, but most cause the output protection features of the commercial amplifier to become ineffective when they are employed. A key feature of this invention is the ability to retain the protection characteristics of the commercial amplifier even though the current available to the load has been greatly increased. A further feature of the invention is to accomplish this with a small number of components, leading to a very high power amplifier with extremely low manufacturing cost.